A basic dry film for applying a photoimageable layer to a substrate, e.g., in the case of a primary resist to a blank for forming a printed circuit board or in the case of a solder mask-forming composition to a printed circuit board, consists of a support or cover sheet, a photoimageable composition layer and a protective sheet. The support sheet is sufficiently flexible to allow the dry film to be rolled, yet has sufficient rigidity to maintain a generally flat configuration when unrolled; this is usually a polyester sheet, e.g., polyethylene terephthalate. The protective sheet overlies the opposite side of the photoimageable composition layer and allows the dry film to be rolled without the photoimageable composition layer sticking to the underside of the support sheet. This invention is particularly directed to photoimageable compositions which are negative-acting and which are developable in alkaline aqueous solution. Primary imaging photoimageable compositions of this type are described, for example, in U.S. Pat. No. 4,239,849, the teachings of which are incorporated herein by reference. Solder mask-forming compositions of this type are described, for example, in U.S. Pat. No. 4,789,620 , the teachings of which are incorporated herein by reference. By solder mask is meant herein a hard, permanent layer which meets the abrasion tests as defined in IPC-SM-840A, Table 12, Summary of Criteria for Qualification/Conformance (Institute for Interconnecting and Packaging Electronic Circuits). Photoimageable compositions developable in alkaline aqueous solution comprise a polymeric binder having sufficient carboxylic acid functionality to be soluble in alkaline aqueous solution, a photopolymerizable monomer(s) and a photoinitiator, plus additional components such as thermal stabilizers, fillers, pigments, colorants, etc. When exposed to actinic radiation, the photoinitiator generates free radicals, resulting in polymerization of the monomer. This polymerization renders exposed portions insoluble in alkaline aqueous solution while the unexposed portions remain soluble and are washed away during development in alkaline aqueous solution. Solder-mask forming compositions may also contain an additional chemical system, whereby they may be hardened and made permanent during a post-development cure.
To be used, a dry film is unrolled, the protective sheet is removed, and the tacky photoimageable composition layer is applied to the substrate. In a basic three-layer dry film laminate, the general procedure is to lay artwork on the support sheet and expose the photoimageable composition layer through the support sheet. Then the support sheet is removed, and the photoimageable composition layer is developed.
U.S. Pat. No. 4,530,896 to Christensen et al. and U.S. Pat. No. 4,318,975 to Kuznetsov et al. describe primary imaging dry films which include a layer intermediate the support sheet and the photoimageable composition layer. This intermediate layer is selectively adherent to the photoimageable composition layer, relative to its adherence to the support sheet. The intermediate layer allows the support sheet to be removed prior to exposure. Unlike the photoimageable layer which is generally quite tacky, the intermediate layer may be formed of relatively non-tacky substances. Therefore, artwork can be laid directly on the intermediate layer. Because the intermediate layer can be quite thin relative to the requisite thickness of a support sheet, better resolution is obtainable. Also, the intermediate layer protects the photoimageable layer from oxygen inhibition after the support sheet is removed.
U.S. Pat. Nos. 4,889,790 issued to Roos et al. and 4,992,354 issued to Axon et al., the teachings of which are incorporated herein by reference, teach advantages of solder mask-forming dry films containing such an intermediate layer. Importantly, by allowing removal of the support sheet prior to vacuum lamination, these patents teach that much better conformation of the photoimageable layer to the surface contours of the printed circuit board is achieved.
A number of materials have been proposed for forming the intermediate layer. One commonly used type of material for forming intermediate layers is polyvinyl alcohol, including modified polyvinyl alcohol and polyvinyl alcohol copolymers. Above-referenced U.S. Pat. No. 4,992,354 describes intermediate layers which are mixtures of polyvinyl alcohol or polyvinyl alcohol copolymers with hydroxyethyl cellulose. The intermediate layer is usually selected to be soluble in water so that it is easily and completely removed to allow for rapid development of the underlying photoimageable composition layer.
A problem that has been noted with such intermediate layers is hazing that results from absorption of water vapor in the air. Hazing both reduces light transmission and disperses light that is transmitted, resulting in some loss of photospeed and loss of resolution. In the case of solder mask-forming dry films, the hazing results in reduced gloss in the final product. Lack of gloss in the solder mask is a major cause of poor flux removal property. The present invention addresses the hazing problem and achieves other advantages as set forth hereinafter.